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PSU tests OK but when changing to another one, PC boots

Posted on 2013-11-30
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Last Modified: 2013-12-01
My PC stopped working the other day. Couldn't turn it on anymore. There was a green light that turned on, on the motherboard, other then that, nothing. No fans, no beeps, nothing on the display. Tested the PSU with a PSU tester, it tested OK. Every voltage and measurement within its range. Still when changing to a different PSU, everything boots and starts as normal. Any known possible explanations to this?
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Question by:itnifl
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Expert Comment

by:Insoftservice
ID: 39686861
Check whether your monitor is attached properly to your display card.
Did your whether processor is attached properly
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by:itnifl
ID: 39686924
If any of those were not attached properly, I would get the same result with both PSU's. Both PSU's test fine, but computer does not boot with the old one. Re-tested several times.
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John Hurst earned 800 total points
ID: 39686950
Every voltage and measurement within its range

Any decent Voltmeter would not present a load to the power supply outputs. This is normal.

So then it is within reason that the old problem supply was unable to supply the required load but when you measured it, it said the voltage was OK.

I think this is what you are seeing.

... Thinkpads_User
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Assisted Solution

by:willcomp
willcomp earned 700 total points
ID: 39686951
I've seen the same thing several times - PSU tests OK but will not boot system. Generally, that means output (watts) on the one of the rails has dropped and there is not enough wattage to boot the PC. Voltages are in range, but wattage is insufficient.
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Expert Comment

by:garycase
ID: 39687168
As willcomp noted, this simply means the voltages are okay, but the current capacity of the old PSU has deteriorated to where it can no longer power your system.    I'd simply toss the old power supply.    Just be glad it was such a simple problem :-)
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by:John Hurst
ID: 39687172
These are pretty much the same answers as I gave earlier when I said (a) a voltmeter does not load a circuit and (b) the problem supply was not able to provide the required current.

... Thinkpads_User
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by:willcomp
ID: 39687561
@thinkpads_user - note times on our responses. I was typing when you posted your answer.
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by:nobus
ID: 39687817
when i test a PS, i always apply a load to the different voltage rails : 12 V, 5 V, and 3.3 V
i made a rig of car lamps for 12 V loads, and use resistor chains for the others.

another cause that it does not start, can be bulging or leaking capacitors inside the PS
aften you have to repalce only a couple to get it working again
here's how they look :
c
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Author Closing Comment

by:itnifl
ID: 39687838
Thanks guys! This is great. i'll just toss the old one and use the new one. I don't want to spend time trying to repair it. I am not familiar with such work, would end up spending more time then interests me, and therefore just opt for the simplest solution :)
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by:John Hurst
ID: 39688039
@itnifl - Thanks for the update. In this case, replacement is probably the right answer.

.... Thinkpads_User
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by:nobus
ID: 39688175
nearly all bad PS's  i had in have been repaired
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